Over the last two decades, three important observations have been made regarding the pathogenesis of nontuberculous mycobacterial infections. 1) In patients infected with HIV, nontuberculous mycobacterial infections often occur when the CD4+ T-lymphocyte number falls below 50/mm3. This suggested that specific T cell products or activities were required for mycobacterial resistance. 2) An association was noted between pulmonary nontuberculous mycobacterial infections and a particular body habitus, predominantly in post-menopausal women (pectus excavatum, scoliosis, mitral valve prolapse). 3) Multiple defects have been found involving the interferon gamma synthesis and use pathways in patients with disseminated nontuberculous mycobacterial infection without HIV, suggesting this is a critical pathway for host defense against these organisms.